Workers who exercise regularly earn 9% higher pay on average than those who don’t, according to a study published in the June issue of the Journal of Labor Research. Past studies have linked exercise with higher earnings, but the direction of cause and effect wasn’t clear. If gym-goers are more disciplined than their slothful coworkers, they might earn higher pay simply because they’re better workers, not because they exercise.

In an effort to cut through the confusion, Vasilios Kosteas of Cleveland State University, the study’s author, used a statistical technique called propensity-score matching. The idea is to score each study subject on whether they fit the profile of someone who exercises, based on factors like age, education level, and whether he or she played sports in high school. By comparing subjects with similar scores, only some of whom exercise, Mr. Kosteas says his study indicates that exercise leads to higher earnings — although he also says follow-up studies are needed to know for sure.

Exercise has been shown in other studies to boost mental function and energy levels and improve mood. In that respect, it’s possible that it makes workers more valuable to employers.If it’s any motivation for workers, the study suggests time at the gym pays for itself, and then some.